The internal structures of houses and other buildings are commonly protected from environmental elements by exterior siding, roofing and trim materials. These siding, roofing and trim materials are typically planks, panels or shingles composed of wood, concrete, brick, aluminum, stucco, wood composites, or fiber-cement composites. Some common fiber-cement composites are fiber-cement siding, roofing, and trim which are generally composed of cement and unbleached wood pulp, and, optionally, silica sand, synthetic fibers and various additives. Fiber-cement products offer several advantages over other types of materials, such as wood siding, roofing and trim because fiber-cement products are weatherproof, relatively inexpensive to manufacture, fire-resistant, and invulnerable to rotting or insect damage.
Most commercial fiber-reinforced cement siding products are made using the Hatschek process. The Hatschek process was initially developed for the production of asbestos cement composites, but is now also used for the manufacture of non-asbestos, synthetic and cellulose fiber reinforced cement composites. Non-asbestos, air-cured fiber cement products require synthetic fibers as reinforcing fibers and refined cellulose or equivalent fibers as filtration fibers. Synthetic fibers alone cannot perform the filtration function and therefore require the addition of cellulose fibers for this purpose.
In the Hatschek process, bales of bleached or unbleached cellulose pulp fibers are re-pulped in water to provide substantially singulated fibers. The re-pulped fibers are refined and then mixed with cement and additives such as calcite, and optionally synthetic fibers, silica sand, clay, and other additives to form a mixture. The type of additive will depend, in part, on the type of curing that will be used. An air-cure or natural curing process will often use calcite, calcium carbonate, as an additive and synthetic fibers like polyvinyl alcohol (PVA) fibers as reinforcing fibers. An autoclave curing process will usually use silica sand as an additive.
A thin layer of fiber-cement mixture is deposited on a felt band substrate and vacuum dewatered. This process is repeated until a number of layers are formed to obtain the final thickness and to provide a layered product. Depending on the desired product and specification the product may then be pressed. The product is then cured to form a fiber reinforced cement matrix in sheet form. The curing may be accomplished by air drying or natural curing in a humid environment, or through autoclaving. A natural curing process may take 21 to 28 days. It can be accelerated by the addition of a high humidity environment at elevated temperature not higher than 80° C. The material may be used for siding, roofing or trim. The siding form may have the appearance of standard beveled wood siding. The roofing form may have the appearance of standard roofing materials such as shingles, tile, slate or full profiled sheets.
In the original Hatschek process asbestos fibers were the fibers of choice. In natural curing, asbestos fibers acted as both a reinforcing fiber and as the filtration fiber. A filtration fiber acts as a filter medium in the cement mixture slurry during the drainage process on the forming or sieve wire (also known as sieve cylinder) to help retain cement and additive particles while the excess water is being removed from the cement suspension. If there is no filter medium then a great deal of the solids from the slurry will be lost with the water during the drainage process. The purpose of the filter medium is to retain the cement mixture within the product while removing the water. The cement mixture will form around and attach to the fibers during the drainage process. Filtration fibers aid drainage by trapping particles of cement and other ingredients in the cement mix without greatly slowing down the formation process on the wire.
The fiber cement board manufacturers target high strength combined with good flexibility in the cement board. These properties are usually measured by a 3-Point Flexure test (similar to ASTM C-1185). Strength is indicated by the modulus of rupture (MOR) of the board. Flexibility is shown by the deflection of the board at maximum load. Maximum load is the amount of force that can be applied to the board before it breaks. Deflection at maximum load is how far the board deflects from the horizontal plane of the board before breaking in 3-Point bending. These measurements are illustrated in FIG. 1.
The asbestos fibers primary function was to reinforce the composite while aiding in the filtration process during manufacture of the board. Health and safety issues are eliminating asbestos fibers from use in fiber cement manufacture. Synthetic fibers, such as polyvinyl alcohol (PVA) fibers have replaced asbestos fibers. Synthetic fibers, however, do not act as filtration fibers. Synthetic fibers do not deter or prevent the cementitious material from passing through the wire with the water. Consequently, highly beaten and highly fibrillated, usually unbleached, cellulose fiber has been combined with synthetic fiber to provide filtration capability.
For naturally cured fiber cement board, PVA reinforcement fibers in combination with highly refined cellulose fibers have been used in place of asbestos fibers. PVA fibers may be used to improve the toughness (calculated by dividing the energy to break by the volume of the board in 3-Point bending test) of the cement product. The PVA fibers provide acceptable modulus of rupture, maximum load and deflection at maximum load. The highly refined cellulose fibers provide filtration. A typical fiber amount is 4 to 5% by weight refined cellulose wood pulp fibers and 1.5 to 3% by weight PVA fibers. The weight percent is based on the dry weight of the ingredients for the cement product, including the fiber, and indicates the amount of fiber in the cement mix.
The replacement of asbestos fibers with highly beaten cellulose fibers as the filtration fiber may require the use of flocculants as one of the additives. The natural affinity of cellulose fibers for the minerals used in fiber cement manufacture is much lower than asbestos. Therefore, flocculants are required for mineral retention, dewatering, formation and machine efficiency. Flocculent selection and optimization formulations are considered to be a competitive advantage by fiber cement manufacturers and kept by each as a trade secret. The typical flocculants are anionic polyacrylamides or phenol-formaldehyde resin and poly(ethylene oxide).
Other commonly used fiber cement manufacturing processes known to those skilled in the art and which use PVA fibers are: the Magnani process, extrusion, injection molding, hand lay-up, molding and the Mazza pipe process.
A drawback to the use of PVA fibers is the high cost of the fibers and the potential lack of availability of the fiber, as well as their inability to filter the slurry in the process causing major solids loss if filtration fibers such as highly refined cellulose fibers were not used in combination with the PVA fibers.
Other fibers must be comparable with PVA/wood pulp fiber mixes in terms of toughness, modulus of rupture, maximum load and deflection at maximum load, and filtration, if they are to be considered for use in fiber cement board.